To keep rodents out of your rig here are a few things you can do:
The mothballs or peppermint (extract will work best) may work as a deterrent.
Completely sealing the underside will keep them out of the unit (they want to be warm and dry too!).
You can fill any openings in the under-carriage with steel wool. They will stay away from that also.
1/4 inch wire mesh will keep rodents out, but be careful about covering any appliance vents or openings as these are designed to let in a specific air or vent a certain amount of exhaust.
Make the area uninviting to them by removing weeds or tall grass from under the rig and keep trash containers tightly sealed.